Process for the production of a cyanic acid-ammonia gas mixture

ABSTRACT

The present invention is directed to a method of producing a gaseous mixture of cyanic acid and ammonia of low cyanuric acid content by introducing molten urea in a fluidized bed of at least 330* C. wherein it is heated to 300* to 480* C. within less than 1 second, the urea being introduced through the injection point with the latter being surrounded concentrically by a blowing gas issuing around the molten urea at a speed between 20 m./sec. to 100 m./sec.

United States Patent Schmidt et al.

[151 3,652,209 51 Mar. 28, 1972 Alfred Schmidt; Ferdinand Welnrotter;Walter Muller, all of Linz Donau, Austria Inventors:

Assignee: Osteneiehische Stickstoiiwerke Aktiengesellschaft, Linz Donau,Austria Filed: Feb. 26, 1969 Appl. No.: 802,685

Foreign Application Priority Data Mar. 1, 1968 Austria ..l988/68 U.S.Cl. ..23/151, 23/193 Int. Cl ..C01c 3/00,C0lc 1/00 Field oiSearch..23/151, 193; ZZZ/566,544;

[56] References Cited UNITED STATES PATENTS 1,869,688 8/1932 l-leimann..7l/36 FOREIGN PATENTS OR APPLICATIONS 534,973 12/1956 Canada ..23/l5l974,145 11/1964 GreatBritain Primary Examiner0scar R. Vertiz AssistantExaminer-I-loke S. Miller Attomey-wenderoth, Lind & Ponack [57] ABSTRACTby a blowing gas issuing around the molten urea at a speed between 20m./sec. to 100 m./sec.

4 Claims, No Drawings PROCESS FOR THE PRODUCTION OF A CYANIC ACID-AMMONIA GAS MIXTURE This invention relates to a process for theproduction of a cyanic acid-ammonia gas mixture from urea.

The manufacture of a cyanic acid-ammonia gas mixture by thermaldecomposition of urea blown into a fluidized bed of inert material isknown by British Pat. No. 974,145. Various I methods of introducing theurea into the fluidized bed are possible.

Thus, for example, it is known to introduce the urea in a solid orliquid form into the decomposition apparatus. It is important for theprocess that the urea should be evaporated rapidly and without leaving aresidue since otherwise cyanuric acid forms, so that the inert particlesof the fluidized bed are stuck together by the solid cyanuric acid,which makes the plant incapable of functioning. This diflicultymanifests itself to a particular extent in the large scale industrialuse of the gasification of urea. In order to ensure perfect evaporationof the urea even in large scale industrial operation, the minimumtemperature of above 320 C. must be reached extremely rapidly and thedecomposition of the urea must proceed so rapidly that cyanuric acidcannot form. It has been suggested for this purpose to blow urea in asolid form, and in particular in the form of prills, into the fluidizedbed from below.

However, a process for the manufacture of melamine from urea is alsoknown in which the more conveniently handled liquid form of urea isblown at supersonic speed, using ammonia as the spraying gas, through anozzle laterally into the catalyst fluidized bed. The spraying gas andthe liquid urea are mixed immediately before blowing through the nozzleaperture. However, this high gas speed on the one hand demands aconsiderable expenditure of power and on the other hand producesenormous abrasion of the solid particles in the fluidized bed.

It has now been found that in the decomposition of urea to give cyanicacid and ammonia by atomizing liquid urea it is also possible to work atgas speeds below the speed of sound if the gasification of liquid ureais conducted in such a way that the urea melt does not come into contactwith surfaces which are below the reaction temperature.

Accordingly the present invention provides a process for the productionof a cyanic acid ammonia gas mixture of low cyanuric acid content whichcomprises blowing molten urea into a fluidized bed of inert materialwhile heating the blown urea to a temperature of 300 to 480 C. withinless than 1 second, the fluidized bed being maintained at a temperatureof at least 330 C., and the molten urea being introduced simultaneouslywith a blowing gas which issues concentrically around the injectionpoint or just before the injection point of the molten urea, the blowinggas, when issuing precisely around the molten urea, having a speed of 40m./sec. to 100 m./sec., and when issuing just before the injection pointof the molten urea having a speed of m./sec. to 100 m./sec., andcontinuously withdrawing the mixture of cyanic acid, blowing gas andammonia which forms in the fluidized bed.

The process according to the invention is preferably carried out byintroducing the molten urea from below, by means of a two-substancenozzle, into the fluidized bed, consisting of finely granular inertmaterial, in a heated fluidized bed reactor.

Ammonia is preferably employed as the blowing gas for atomizing theliquid urea and as the carrier gas for maintaining the fluidized bed.However, nitrogen, carbon dioxide and other gases which are inerttowards the reagents are also suitable. The blowing gas as well as thecarrier gas must be introduced in an anhydrous state since, as is known,water would immediately decompose the cyanic acid with the formation ofcarbon dioxide and ammonia.

When atomizing liquid urea care must be taken that the issuing jet ofliquid urea is always surrounded by a jacket of blowing gas. If theblowing gas issues precisely around the injection point of the urea, ahigher gas speed of the blowing gas is necessary to ensure trouble-freeoperation of the urea decomposition than in cases where the injectionpoint of the blowing gas is in front of that of the urea melt. Themixing of the blowing gas with the liquid urea must only take 'placeafter leaving the nozzle.

A suitable device for carrying out the process comprises a fluidized bedreactor having a bottom plate provided with holes and an inlet forintroducing the reagent from below, the said inlet comprising a bore fora two-substance nozzle passing through the bottom plate of the fluidizedbed, the nozzle consisting of a central tube for introducing the moltenurea and a jacket tube arranged concentrically to the central tube, withthe central tube, at the end projecting into the reaction space, eitherending at the same level as the jacket tube or projecting a fewmillimeters beyond the latter. The above device renders mixing withinthe nozzle and contact with the nozzle wall impossible.

The following Examples illustrate the invention in more detail.

EXAMPLES An externally heated fluidized bed reactor of 200 mm. diameterand 500 mm. height is filled to a height of l mm. with sand of 0.1 to0.3 mm. diameter. 8,000 llhour of ammonia at a temperature of 20 C. areblown in as the carrier gas through a frit at the bottom of the reactor.2 kg./hr. of molten urea at a temperature of C., and the quantity ofammonia specified in Table l, acting as a blowing gas and at atemperature of C., are introduced from below, via a nozzle of the typedefined below, through a central bore in the frit.

The temperature of the fluidized bed is kept at 350 to 370 C.

The two-substance nozzles employed, having a central liquid entry, areof the following dimensions:

Diameter of the jacket pipe: internal: 4.4 mm. I external: 7.3 mm.Diameter of the central pipe: internal: L85 mm.

external: 3.3 mm.

Nozzle 1: Central pipe 2 mm. longer than jacket pipe. Nozzle 2: Centralpipe and jacket pipe of equal length.

approx. 63

If the nozzles described are operated out of the reactor, it is seenthat the external jacket pipe is in the case of Nozzle 1 not wetted withmolten urea in any of the three experiments, whilst in the case ofNozzle 2 it is wetted at its tip when operated with 500 l/hour ofammonia gas.

We claim:

1. In a process for the production of a cyanic acid-ammonia gas mixtureby the introduction of urea into a fluidized bed of inert materialwherein it is heated to a temperature of 300 to 480 C. within less than1 second, the improvement which comprises maintaining the fluidized bedat a temperature of at least 330 C. and simultaneously introducingthrough concentric nozzles at the bottom of said fluidized bed, moltenurea through the inner nozzle of said concentric nozzles and a blowinggas through the outer nozzle of said concentric nozzles, the innernozzle ending at the same level as the outer nozzle so that the blowinggas issues precisely around the injection point of the urea, saidblowing gas having a speed of 40 m./sec. to 100 m./sec., the mixture ofcyanic acid, blowing gas and ammonia which forms in the fluidized bedbeing continuously withdrawn.

2. ln a process for the production of a cyanic acid-ammonia gas mixtureby the introduction of urea into a fluidized bed of inert materialwherein it is heated to a temperature of 300 to 480 C. within less than1 second, the improvement which comprises maintaining the fluidized bedat a temperature of at least 330 C. and simultaneously introducingthrough concentric nozzles at the bottom of said fluidized bed, moltenurea through the inner nozzle of said concentric nozzles and a blowinggas through the outer nozzle of said concentric nozzles, the innernozzle projecting a few millimeters beyond the outer nozzle so that theblowing gas issues at a point just before the injection point of theurea, said blowing gas having a speed of 20 m./sec. to m./sec., themixture of cyanic acid, blowing gas and ammonia which forms in thefluidized bed being continuously withdrawn.

3. A process according to claim 1 wherein the blowing gas is ammonia.

4. A process according to claim 2 wherein the blowing gas is ammonia.

2. In a process for the production of a cyanic acid-ammonia gas mixtureby the introduction of urea into a fluidized bed of inert materialwherein it is heated to a temperature of 300* to 480* C. within lessthan 1 second, the improvement which comprises maintaining the fluidizedbed at a temperature of at least 330* C. and simultaneously introducingthrough concentric nozzles at the bottom of said fluidized bed, moltenurea through the inner nozzle of said concentric nozzles and a blowinggas through the outer nozzle of said concentric nozzles, the innernozzle projecting a few millimeters beyond the outer nozzle so that theblowing gas issues at a point just before the injection point of theurea, said blowing gas having a speed of 20 m./sec. to 100 m./sec., themixture of cyanic acid, blowing gas and ammonia which forms in thefluidized bed being continuously withdrawn.
 3. A process according toclaim 1 wherein the blowing gas is ammonia.
 4. A process according toclaim 2 wherein the blowing gas is ammonia.